The present invention relates generally to gas turbine engine compressor case designs. More particularly, one form of the present invention relates to a compressor case including a mechanical arrangement that provides both passive tip clearance control and endwall ovalization control. Although the present invention was developed for application in a gas turbine engine compressor, applications in other components of the gas turbine engine are contemplated herein.
Tip clearance has been shown to be a significant factor influencing compressor performance. Current design trends attempt to control tip clearance to span ratios on the order of about one to two percent at the rear of the compressor. Tip clearance levels above this range can seriously affect compressor efficiency and stall margin.
A variety of compressor case designs have been employed in previous gas turbine engines over the years. These designs include a full hoop compressor case, a horizontally split compressor case, a single wall compressor case, a dual wall compressor case and a triple wall compressor case. Each of these prior designs has been found to affect tip clearance in unique ways.
A single wall horizontally split compressor case has been widely utilized in gas turbine engine compressors for its simplicity and low cost. The horizontally split compressor case introduces endwall ovalization into the system due to the non-axisymmetric nature of the casing. The single wall case leaves only the casing material selection (and subsequent coefficient of thermal expansion) as a design variable affecting tip clearance. As a result of these limitations, the majority of current production engines avoid single wall horizontally split compressor cases.
Dual wall compressor cases have been utilized in many military and commercial gas turbine engine applications. The dual wall compressor cases are typically configured with a split outer flowpath hung off of the outer compressor casing through hooks and in some cases a wishbone shaped diaphragm. The outer casing can be of a full hoop type, see for example U.S. Pat. No. 5,180,281, or of the horizontally split type. While dual wall compressor cases have provided improved tip clearance and ovalization control relative to a single wall compressor case, additional improvement has been sought through the application of complex triple wall compressor cases.
Although many of the prior compressor case designs have been steps in the right direction, there still remains a need for additional improvement. The present inventions satisfy this need in a novel and non-obvious way.